Conventionally, to connect a phone to a main device from a place of the main device to a geographically distant place, it is necessary to install a necessary number of IP terminals 903-1, 902-2, . . . and 902-n as illustrated in FIG. 1 and connect each of IP terminals 903-1, 902-2, . . . and 902-n to an upper main device through an IP network.
Further, although there was a technique of connecting a digital terminal to an upper main device through a digital network other than the IP network, there is no technique of connecting the digital terminal to the upper main device through the IP network.
PTL 1 discloses connecting a terminal housing device to a communication device connected to an IP network and a line switching network through a digital subscriber line and connecting a terminal device to the terminal housing device.    {PTL 1} JP-A-2010-124456